1. Field of the Invention
The present invention relates to a synchronization circuit of an RDS decoder, and more particularly, a subcarrier recovery circuit and symbol timing recovery circuit of an RDS decoder and related methods thereof.
2. Description of the Prior Art
Radio Data System (RDS) is a standard from the European Broadcasting Union for sending digital information using conventional FM (frequency modulation) radio broadcasts. Radio Broadcast Data System (RBDS) is the official name used for the North American version of RDS, but is also commonly referred to as RDS. The RDS system standardizes several types of information transmitted and uses a 57 kHz subcarrier, which was chosen for being the third harmonic (3×) of the 19 kHz pilot tone for FM stereo.
To decode the RDS signal, a typical radio receiver first locks onto the received pilot tone and then calculates the third harmonic of the pilot tone frequency (19 kHz) to find the RDS subcarrier frequency (57 kHz).
If the transmitter of the radio signal employs two separate modulators, however—that is, one FM modulator for the audio signal and another modulator for the RDS signal—the clock signal feeding to each modulator may be slightly different from one another. The undesired result is that the RDS subcarrier may not be exactly the third harmonic of the pilot tone. For example, if the pilot tone is substantially under the typical 19 kHz and the RDS subcarrier is slightly higher than the normal 57 kHz, a radio receiver may have difficulties locking onto the RDS subcarrier signal based on the received pilot tone. This difficulty is also possible when each modulator experiences differing frequency drift, particularly in opposite directions.
Consequently, the radio receivers experiencing the above problems will exhibit poorer reception of the RDS signal, and reduced performance in providing RDS data to the user.